1. Field
This disclosure relates to linear polarization to circular polarization converters for use in coaxial waveguides.
2. Description of the Related Art
Satellite broadcasting and communications systems commonly use separate frequency bands for the uplink to and downlink to and from satellites. Additionally, one or both of the uplink and downlink typically transmit orthogonal right-hand and left-hand circularly polarized signals within the respective frequency band.
Typical antennas for transmitting and receiving signals from satellites consist of a parabolic dish reflector and a coaxial feed where the high frequency band signals travel through a central circular waveguide and the low frequency band signals travel through an annular waveguide coaxial with the high-band waveguide. An ortho-mode transducer (OMT) may be used to launch or extract orthogonal TE11 linear polarized modes into the high- and low-band coaxial waveguides. TE (transverse electric) modes have an electric field orthogonal to the longitudinal axis of the waveguide. Two orthogonal TE11 modes do not interact or cross-couple, and can therefore be used to communicate different information. A linear polarization to circular polarization converter is commonly disposed within each of the high- and low-band coaxial waveguides to convert the orthogonal TE11 modes into left- and right-hand circular polarized modes for communication with the satellite.
Converting linearly polarized TE11 modes into circularly polarized modes requires splitting each TE11 mode into two orthogonally polarized portions and then shifting the phase of one portion by 90 degrees with respect to the other portion. This may conventionally be done by inserting two or more dielectric vanes, oriented at 45 degrees to the polarization planes of the TE11 modes, into the waveguide as described in U.S. Pat. No. 6,417,742 B1. However, assembling the dielectric vanes at the precise angle within the waveguide can be problematic. Errors in assembling the dielectric vanes can result in imperfect polarization conversion and cross-talk between the two orthogonally polarized TE11 modes.
Throughout this description, elements appearing in figures are assigned three-digit reference designators, where the most significant digit is the figure number where the element was first introduced and the two least significant digits are specific to the element. An element that is not described in conjunction with a figure may be presumed to have the same characteristics and function as a previously-described element having the same reference designator.